1. Field of the Invention
The present invention relates to apparatuses and methods for acquiring a time waveform of terahertz waves. The present invention particularly relates to a terahertz wave measuring system (which may be called a THz-TDS system or THz-time domain spectroscopy system) and method which measure terahertz waves in the time domain.
2. Description of the Related Art
Terahertz waves are electromagnetic waves that have a component in an arbitrary frequency band in the range from 0.03 THz to 30 THz. In such a frequency band, many distinct absorption bands are present due to the structures and/or states of various substances including biomolecules. The characteristic is used in examination technologies developed for nondestructive analysis or identification of a substance. It has been expected that terahertz waves would be applied to imaging technologies so as not to use X-rays, and applied to high-speed communication technologies and the like.
The time waveform of terahertz waves is subpicosecond pulse shape in many cases. Since such pulses are difficult to acquire in real time, a THz-TDS system performs sampling measurement using excitation light having a pulse width of the femtosecond order. The sampling can be implemented by adjusting the difference between times taken for the excitation light beams to reach a generating portion which generates terahertz waves and to reach a detecting portion which detects the waves. For example, a stage (will also be called delay optical portion) may be provided in a propagation path of the excitation light. The stage has a folding optical system, and the time difference may be acquired by adjusting the amount of folding. The generating portion or detecting portion may sometimes be a photoconductor with an antenna pattern having a microgap in a semiconductor film. The following descriptions assume that the generating portion or detecting portion is such a photoconductor.
In recent years, there have been pointed out the influences of swinging of the stage used as the delay optical portion with the improvements in measurement sensitivity of the THz-TDS system. (Refer to Japanese Patent Laid-Open No. 2008-014815). When the stage used as the delay optical portion swings, the optical axis of the excitation light sways. This as a result changes the quantity of light per unit area radiated to the microgap in the photoconductor, which may deteriorate the SN ratio of terahertz signal.
The stage being the delay optical portion as described above may sometimes be a linear-motion stage which moves several tens of millimeters. The linear-motion stage may sometimes be a combination of a stepping motor and a micro ball screw to move a table. Since such a stage mechanically moves a table, a surface of the table may be displaced from a base plane, on which the stage is placed, due to non-uniformity of the thread pitch of the ball screw and the like. Under certain environmental conditions such as those of the ambient atmosphere around the delay optical portion or under certain installation condition such as location of the delay optical portion, the surface of the table may also be displaced from the base plane. Displacement may be caused by distortion of the housing and/or member supporting the delay optical portion and/or caused by the stage vibrating at a frequency inherent to the place where the stage is located. When the displacement due to the movement of the table causes periodic swinging of the delay optical portion, the optical axis of the excitation light periodically sways. Thus, a periodical signal component is superposed on the time waveform of terahertz waves. Performing a Fourier transform on the time waveform results in a spectrum of the terahertz wave detected by the detecting portion and a pseudospectrum due to the swinging of the delay optical portion. For example, the superposition of a frequency component of a swing being several hundred Hz on a time waveform of terahertz waves may result in a pseudospectrum around 4 to 6 THz. This reduces the measurement band width for a measuring apparatus and may lower the analysis performance.